1. Field of the Invention
The present invention relates to a high-pressure metal halide discharge lamp suitable for a light source used in a light fixture or a lighting device.
2. Description of the Related Art
A high-pressure metal halide discharge lamp may be used in any light fixture because of its high luminous efficacy, long life, and good color rendering property. Recently, such a lamp must show high luminous efficacy and long life with a view to saving energy. To increase the luminous efficacy, temperature of a discharge vessel can be raised. The discharge vessel contains an ionizable filling comprising Hg, a rare gas and a metal halide.
Therefore, Hg and the metal halide are able to evaporate quickly. In this case, as the temperature of electrodes in the discharge vessel also becomes higher, the evaporation from the electrodes, which are made of tungsten, increases. This evaporation in the discharge vessel quickly blackens the lamp.
Such a high-pressure metal halide discharge lamp is shown in Japanese Laid Open Patent Application HEI 5-283039. This prior art discloses a technique for preventing quick blackening and improving a lumen maintenance factor of the lamp, by reason of a tip of the electrodes being re-crystallized.
When the conventional lamp having a pair of electrodes is uprightly mounted on a socket, which is provided at an upper side in a light fixture, the temperature of the upper electrode becomes higher than lower electrode. As the evaporation of the electrode at the upper side in the discharge vessel of the lamp increases, it causes the quick blackening. Therefore, the upper and lower electrodes made of doped-tungsten may be useful to obviate the evaporation thereof. Further in this case, the lamp might turn off occasionally. Because the temperature of the lower electrode remains low for the reason of no heat convection in the discharge vessel, thermionic electrons do not appropriately emit from the lower electrode.
It is an object of the invention to provide a high-pressure metal halide discharge lamp in which blackening and extinguishing are obviated.
According to the invention, a high-pressure metal halide discharge lamp includes a sealed light-transmitting discharge vessel, first and second electrodes disposed in the discharge space and a pair of conductive wires connected to the respective electrodes.
The sealed light-transmitting discharge vessel has a pair of seals and envelops a discharge space, which has a gas filling comprising a rare gas and metal halides. The first electrode, which is disposed in the discharge space at one side, is made of a metal having a high melting point and having an emitter. The second electrode without an emitter is disposed in the discharge space at the other side. The second electrode is also made of a metal having a high melting point. A pair of conductive wires connected to the respective electrodes, are located in the respective seals and extend from the discharge vessel.
In accordance with the present invention, a lighting apparatus includes a high-pressure metal halide discharge lamp as described above, a luminarie housing having a ballast, and a socket connected to the discharge lamp cap.
These and other aspects of the invention will be further described in reference to the following drawings and detailed description of the invention.